Intumescent degradation

Protective Coatings. Some flame retardants function by forming a protective Hquid or char barrier. These minimize transpiration of polymer degradation products to the flame front and/or act as an insulating layer to reduce the heat transfer from the flame to the polymer. Phosphoms compounds that decompose to give phosphoric acid and intumescent systems are examples of this category (see Flame retardants, phosphorus flame retardants). 

Specimens of NR ABS/(Octa -I- AO) heat-treated at 350°-400°C developed brittleness of connected pores, whereas VO ABS (Octa -1- AO -I- EPDM), similarly treated, was tougher with large elongated pores about twice the size of the non-treated specimen. Such behavior suggests an intumescent effect of EPDM, i.e. the development of a thick porous surface layer, inhibiting the diffusion of flammable products of plastic degradation towards the gas phase and heat transfer into the plactic mass. 

Camino, G., Costa, L., and Trossarelli, L. 1985. Study of the mechanism of intumescence in fire retardant polymers Part V—Mechanism of formation of gaseous products in the thermal degradation of ammonium polyphosphate. Polym. Deg. Stab. 12 203-211. 

Bourbigot, S., Le Bras, M., and Delobel, R. 1995. Fire degradation of an intumescent flame retardant... 

Li, B. and Xu, M. 2006. Effect of a novel charring-foaming agent on flame retardancy and thermal degradation of intumescent flame retardant polypropylene. Polym. Deg. Stab. 91 1380-1386. 

Vannier, A., Duquesne, S., Bourbigot, S., Castrovinci, A., Camino, G., and Delobel, R., The use of POSS as synergist in intumescent recycled poly(ethylene terephthalate), Polym. Degrad. Stab. 2008, 93, 818. 

Bourbigot, S., Le Bras, M., Delobel, R., Breantb, P., and Tremillon, J.M., 4A zeolite synergistic agent in new flame retardant intumescent formulations of polyethylenic polymers-study of the effect of the constituent monomers, Polym. Degrad. Stab. 1996, 54, 275. 

Moreover, in the stabilization phase of the intumescent structure, the change in the viscosity of the charred material under stress may explain the loss of the protective character of the intumescent shield. Indeed, if the shield becomes too hard, the creation and the propagation of cracks leading to a rapid degradation of the material occurs.33... 

At 360°C-370°C, an important increase in the apparent initial viscosity begins that corresponds to the complete degradation of the initial material and to the carbonization of the system. The foamed material appears to be constituted by solid particles only. From 450°C-460°C, the viscosity value is more stable and increases slightly. At this temperature, a char oxidation/degradation probably starts. Complementarity, the viscosity can be analyzed versus temperature and versus time (Figure 10.10) in order to better visualize the mechanical and thermal stabilities of the protective intumescent layer and to have a better understanding of the carbonization process. 

Whereas no expansion is observed for the virgin resin, the four coatings show maximum expansion between 300°C and 350°C. The expansion is attributed to the evolution of volatile degradation products of the resin as well of the intumescent additives, which are trapped in the structure. 

Duquesne, S., Le Bras, M., Jama, C., Weil, E.D., and Gengembre, L. 2002. X-ray photoelectron spectroscopy investigation of fire retarded polymeric materials Application to the study of an intumescent system. Polymer Degradation and Stability 77(2) 203-211. 

Anna, P., Marosi, Gy., Csontos, I., Bourbigot, S., Le Bras, M., and Delobel, R. 2001. Influence of modified rheology on the efficiency of intumescent flame retardant systems. Polymer Degradation and Stability 74(3) 423M-26. 

Duquesne, S., Magnet, S., Jama, C., and Delobel, R. 2005. Thermoplastic resins for thin film intumes-cent coatings—Towards a better understanding of their effect on intumescence efficiency. Polymer Degradation and Stability 88(l) 63-69. 

M. Le Bras, S. Bourbigot, and B. Revel, Comprehensive study of the degradation of an intumescent EVA-based material during combustion, J. Mater. Sci., 1999, 34 5777-5789. 

F. Laoutid, L. Ferry, E. Leroy, and J.M. Lopez-Cuesta, Intumescent mineral fire retardant systems in EVA copolymer Effect of silica particles on char cohesion, Polym. Degrad. Stabil., 2006, 91 2140-2145. 

The same method is applicable to control critical temperature where the reaction between polyphosphates and polyols takes place. For example, melamine polyphosphate-polyol system, which is stable at the degradation temperature of PP, has been activated for application in polyolefins [47], An activating effect is ascribed also to the char-forming polymers in intumescent nanocomposites, according to the blending approach [48],... 

The efficiency of intumescent fire retardants could be enhanced by interlayers that deliver the active components to the surface (shown by two examples). The fire-retardant additives, delivered to the surface at early stage of combustion, accelerate the formation of protecting surface layer that hinders the degradation of the underlying material. This coating structure could be reinforced by an interlayer of ceramizing capability (e.g., polyborosiloxane). Phosphorus-free intumescent fire-retardant system could be formed by using such additive. 

Wang, D. Y Liu, Y Ge, X. G Wang, Y. Z Stec, A., Biswas, B Hull, T. R and Price, D. 2008. Effect of metal chelates on the ignition and early flaming behaviour of intumescent fire-retarded polyethylene systems. Polymer Degradation and Stability 93 1024—30. 

Camino G, Costa L, Trossarelli L. Study on the mechanism of intumescence in fire retardant polymers. 1. Thermal-degradation of ammonium polyphosphate pentaerythritol mixtures. Polym. Degrad. Stab. 1984 6 243-252. 

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